1. Field of the Invention
The present invention relates to a semiconductor memory device and more particularly to a storage capacitor structure of a dynamic random access memory (DRAM) and a method of producing the same.
2. Description of the Prior Art
A prior art semiconductor memory device is disclosed, for example, in Japanese Laid-Open Patent Publication No. 62-219659.
FIG. 11 shows a memory cell portion of a prior art semiconductor memory device. As shown in FIG. 11, the semiconductor memory device comprises: a silicon substrate 101; an element isolation film 102 for isolating each element region of the silicon substrate 101; a switching transistor formed at the element region of the silicon substrate 101; and a stacked storage capacitor connected to the switching transistor. The switching transistor has a gate oxide film 103 formed on the silicon substrate 101, a gate electrode 104 formed on the gate oxide film 103, and a drain 105 and a source 106 formed in the element region of the silicon substrate 101. The gate electrode 104 is covered by an interlayer isolation film 107. The storage capacitor has a storage electrode (polycrystalline silicon film) 108 connected to the source 106 of the switching transistor, a dielectric layer including an Si.sub.3 N.sub.4 film 110 and a Ta.sub.2 O.sub.5 film 111, and a plate electrode 109. The storage capacitor is covered by an SiO.sub.2 film 112, and the storage capacitor and the switching transistor are covered by an insulating film 113.
In the prior art semiconductor memory device, a thin native oxide film (not shown) having a thickness of 1.2 to 1.5 nm is inevitably formed at the interface between the polycrystalline silicon film (first electrode) 108 and the Ta.sub.2 O.sub.5 film (dielectric layer) 111. Therefore, when the Ta.sub.2 O.sub.5 film 111 is relatively thick, the effect of the native oxide film on the capacitor region can be ignored, but when the Ta.sub.2 O.sub.5 film 111 is relatively thin, the effect of the native oxide film on the polycrystalline silicon film 108 cannot be ignored. That is, the dielectric constant of the Ta.sub.2 O.sub.5 film 111 (the dielectric constant of a bulk Ta.sub.2 O.sub.5 is approximately 25, but that of a thin film is approximately 12) appears to decrease, and the cell capacitance of the semiconductor memory device decreases as a result.